1. Field of the Invention
This invention relates the field of communications media for transferring data between a source and a plurality of peripheral devices coupled to the source. More particularly, the present invention relates to data transfer along a peripheral device bus between a plurality of peripheral devices and a host computer.
2. Art Background
In the computing industry, it is quite common to transfer data and commands between a plurality of data processing devices, such as for example, computers, printers, memories and the like. The interconnection of computers and other peripheral devices principally developed in the early 1070's with the advent of computer networking systems, which permitted the distribution of access to computing resources beyond the immediate proximity of a main frame computer.
Networks, such as the ARPA network, were developed to provide access by various users to large time-sharing systems and the transfer of data between such systems. In the case of geographically local networks, so-called "local area networks" (LANs) were developed to connect together a collection of computers, terminals and peripherals located, typically in the same building or adjacent buildings, and permitted each of these devices to communicate among themselves or wit devices attached to other networks. Local area networks permit the implementation of distributed computing. In other words, some of the devices coupled to the local area network may be dedicated to perform specific functions, such as file storage, data base management terminal handling, and so on. By having different machines perform different tasks, distributed computing can make the implementation of the system simpler and more efficient.
Presently, networking has only been applied to provide communications between data processing devices, which are machine input devices. However, it would also be useful to provide a networking means to provide communication between a single computer and a plurality of peripheral devices such as human input devices, listen only devices, appliances, etc. Human input devices include keyboards, cursor control devices (such as a "mouse"), and sketch pads, etc. Listen only devices include transaction logs, etc. In the prior art, such devices are attached to a host computer through a port dedicated to each device. Often, additional "cards" are required to allow a peripheral input device to be added. Further, the addition of cards requires that the host computer be powered down, with o mechanism for adding peripheral devices to a live system. Such prior art systems are inefficient since peripheral devices are not generally operated simultaneously. (for example, someone using a mouse is generally not using the keyboard or sketchpad at the same time). Thus, the devices could share a common line to the host computer without creating data traffic problems, eliminating the need for cards.
Prior art networking schemes also include elaborate methods for establishing control of the network to allow a device to transmit. Such systems are not needed for networking of peripheral devices, since only one is generally used at a time. In addition, prior art networking schemes provide for means for attached devices to identify themselves to each other through elaborate "handshaking" schemes. Again, such complexity is not required to connect peripheral devices since there is no need for these devices to identify themselves to other devices, only to the host computer.
Therefore, it is an object of the present invention to provide a communications medium for a plurality of peripheral devices, which provides a simple and efficient means for coupling those devices to a host computer.
It is a further object of the present invention to provide a communications medium by which all such peripheral devices can be coupled to a host computer at a single input.
It is still another object of the present invention to provide a communications medium which provides a means for peripheral devices to indicate a need for servicing to the host computer.
It is yet another object of the present invention to provide a communications medium which provides a means for determining if the communications medium is in use.
It is another object of the present invention to provide a communications medium which allows peripheral devices to be added during operation of the system.
SUMMARY OF THE INVENTION
A communications medium is disclosed including apparatus and methods for transferring data between a plurality of peripheral devices and a host computer. In the preferred embodiment, a plurality of peripheral devices such as human input devices (including mice, keyboards, sketchpads, etc.), appliances, listen only devices, etc., are coupled to a common cable for data transmission and reception of commands. A peripheral device coupled to the cable may signal the host computer when it requires servicing. This peripheral device will continue to request service until the host computer commands it to transmit its data. All peripheral devices of the same generic type (e.g., all keyboards), may have an identical hard wired address used as an identification number. In this manner, the host computer can identify the generic type of device communicating on the cable. If more than one of the same type of device is coupled to the cable (e.g., 2 mice), the host computer will assign new addresses in the status registers of the mice so they can be differentiated.
In the preferred embodiment, a return to zero modulation scheme is used to transmit data and commands over the cable. As a result, a peripheral device will assume a collision if it attempts to transmit a high signal on the cable and the cable is pulled low by another device. In order to simplify the protocol of the system, only the computer can initiate communication.
The present invention permits the addition of peripheral devices to a computer while the computer is in use, without the need to power down the computer system. The present invention can be embodied in a narrow band medium, as well as broad band, fiber optic, infrared and other media.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is block diagram illustrating the networking system of the present ,invention.
FIG. 2 is a timing diagram illustrating the present invention's use, of return to zero encoding.
FIG. 3 illustrates a register of a peripheral device of the present invention.
FIG. 4 is a flow chart illustrating the sequence of operations utilized by a peripheral device to request service by the host computer.
FIG. 5 is a flow chart illustrating the sequence the operations utilized to provide new addresses to devices sharing the same hard-wired address.
FIG. 6 is a timing diagram illustrating a command transaction of the present invention.